A Radio-frequency identification (RFID) tag is composed of a RFID IC and an antenna, wherein the RFID IC can be used to store information such as the product type, location, and date. To read/write information from/into the RFID IC, it is necessary to perform read/write operation to the RFID IC in a contactless manner. Because RFID tag can be used to transmit data in a wireless fashion, it has been widely employed in a variety of fields, such as door access control, ticket vending, antitheft application, logistic management, and pet identification.
Referring to FIG. 1, a conventional antenna for RFID tag is shown. The antenna 1 for using in a RFID tag includes a loop element 11 and a radiating body 12, wherein an annular path is formed between a first feeding point 111 and a second feeding point 112 of the loop element 11. The loop element 11 has an outer side A coupled with the radiating body 12. The radiating body 12 extends outwardly from the side A and bent several times for receiving or transmitting radio waves. The RFID IC (not shown) is connected to the first feeding point 111 and the second feeding point 112. Energy can be transferred to the antenna 1 through the first feeding point 111 and the second feeding point 112. Also, the radio signals received by the antenna 1 can be transferred to the RFID IC through the first feeding point 111 and the second feeding point 112.
The first feeding point 111 and the second feeding point 112 will generate an equivalent inductive reactance therebetween, and the RFID IC will function as a capacitive element. When the RFID IC is connected to the first feeding point 111 and the second feeding point 112, a conjugate-matching compensating effect is generated. Therefore, the RFID IC can effectively transfer the energy to the loop element 11, and thus the loop element 11 can transfer the energy to the radiating body 12 by coupling.
However, the conventional antenna 1 for using in a RFID tag can be used at a single resonant frequency. Therefore, the bandwidth of antenna is small and thus the antenna can be used at a single frequency only. Moreover, the conventional antenna is a non-array type antenna, and its directionality is quite low. This would result in a short reading distance for RFID tag. Therefore, how to develop a high-directional wide-bandwidth antenna for using in a RFID tag is an urgent task.